For wind load, masonry shear wall, moment frame, and cross bracing which is the best option? 1

[johndeng]

to control story drift,
my experience is shear wall is the best,
then cross brace,
the least is moment frame.

Any opnion?

 

[JAE]

Depends on if you are designing a one-story school building or a skyscraper.

Depends on where you are located (local costs vary).
Depends on the function of the building and need for windows, etc.
It depends.

 

[johndeng]

JAE, please correct me:

ok, lets say a story office building. All design options are possible.

Shear wall design will make no story drift,
cross brace will be similar to shear wall, since both shear wall and braced frame are treated as sidesway prevented. But bracing member can extend due to tension, so we do get small drift.
Moment framing is usually treated as sidesway uninhibited. So when we design, we need to seperated drift into nonsway and sway frame drift. Plus column design K value will be larger.

 

[JAE]

If you have a tall enough building - a shear wall will still deflect.

You are asking very general questions without much to go by.
Can you be more specific in your question or situation?

 

[johndeng]

Just want to get a general concept here. We usually only design for less than 3 story building.
I just out my thinking as above. The reason I am confused is that when I read the code book for the R VALUE and Cd value.

R Cd
ordinary moment frames of steel 4.5 4
reinforced concrete shear wall 5.5 5
reinforced masonry shear wall 4.5 4
concentrically-braced frames 5 4.5

 

[UcfSE]

What confuses you about those values?

 

[johndeng]

I thought R value the higher the better, Cd value the lower the better.
But they just confused me.

 

[ukengineer58]

Surely the stabilty option dependd on the building use archtectural requirments programme reasons whether shear walls can fit into archtectural requirments. none is better per say. just more appropriate

 

[structSU10]

The R and Cd values correspond to basically the ductility of the system, and the amplification of deflection due to inelasticity (non-linear material behaavior), respectively. The R value changes based on how an element is detailed, and a high R value means high ductility. R and Cd are related in that if you allow for a lot of inelastic action ( a high R value) your deflection modifier should also be higher. The Cd also varies to the material properties i.e. a concrete shear wall deflects differently in general than a steel X brace.

In other words, they are not the values to use to determine what system is better - a higher R typically is a more expensive system - more special detailing is required, and more design time. Also, if you can, staying in the range of a steel system not detailed for seismic (R=3) is generally a good idea, as you do not have to follow AISC 341 detailing requirements.